The present invention relates to a rubber composition for a pneumatic tire. In particular, the invention relates to a rubber composition that is preferably adoptable in forming a tread rubber part of a studless tire, especially for automobiles such as four-wheel vehicles.
Various efforts and devising have been made and incorporated into the tread rubber part of the studless tire so as to improve contacting between the tread rubber part and ice-covering road surface. For example, the tread rubber part is devised as adjusted to have low rubber hardness at low temperature at near or under 0° C. Moreover, in order to increase tire-tread's friction with the ice covering road surface, there have been proposed to add foamed rubber, hollow particles, glass fibers, lignocellulosic particles such as walnut shell powder, or the like, to the rubber composition for the tread rubber part. These are to achieve scratching on the ice and/or utilizing of undulation caused by dropping out of hard particles or fibers on way of proceeding with wearing away of the tread rubber part. Further, there have been also adopted a method of increasing fillers and oils so as to improve braking performances on wet road and ice-covered road surface. Such method is disadvantageous in increases of rolling resistance (heat generation) and wear resistance of the tire tread rubber part.
JP2002-249619A (Japan patent application publication No. 2002-249619) proposes adding of glass fiber strands or other strands, particles of lignocellulosic or cellulosic materials such as used papers or rice husks so as to achieve removing of water film on the road surface by way of holes or recesses formed by dropping out the fibers or particles. This method is disadvantageous in that high braking performances on wet road and ice-covered road are difficult to be stably maintained and in that wear resistance is decreased due to difficulty in uniform dispersion of the strands or the particles.
In view of the above, it is aimed to provide a rubber composition for a pneumatic tire, which enables to improve contacting of the tire tread, as well as braking performance, on the wet or ice-covering road surface while avoiding increase of wearing out and rolling resistance (heat generation).